The present invention is directed to providing halogen-containing vinyl compounds such as polyvinyl chloride (PVC) resins which are characterized as being thermally stable and exhibiting flame retardancy. While the present invention is applicable to halogen-containing vinyl resins in general such as those heretofore well known in the art, for purposes of discussion only, the invention will be described with reference to PVC.
As is well known in the art, in only a few cases can PVC resins, as received from the manufacturer, be fabricated into products without the addition of a stabilizer. In this regard, PVC is unlike most thermoplastics. Milling or other processing of PVC at elevated temperatures rapidly results in unwanted discoloration. For example, when heated to .about.200.degree. C., a pronounced darkening will occur within thirty minutes. Other degradative effects at elevated temperatures include elimination of hydrogen chloride and irreversible adherence to equipment surfaces. For this reason, it is essential that a stabilizer be incorporated into the PVC resin before it is subjected to elevated temperatures either in fabrication or in its end use.
Stabilizers for PVC are per se well known in the art and accordingly need not be discussed in any great detail. Typical stabilizers for providing heat stability to PVC include Ba/Cd/Zn and phosphite type stabilizers. These and others are discussed in detail, in Handbook of Polyvinyl Chloride Formulating, edited by Edward J. Wickson, John Wiley & Sons (1993), Chapter 9, incorporated by reference herein.
Inherent problems arise when it is desired to include a flame retardant in the resin without adversely affecting both the stability of the resin and its optical qualities. These inherent difficulties may best be illustrated by reference to U.S. Pat. No. 4,608,198 issued to Watanabe et al. Aug. 8, 1986 and assigned to Nissan Chemical.
Watanabe is directed to flame retardants for halogen-containing vinyl compounds such as PVC in which the antimony pentoxide main component does not impair either the transparency or the thermal stability of the PVC. In Col. 1 it is explained that while PVC itself possesses excellent flame retardancy, flexible PVC elasticized with a plasticizer or blended with a combustible resin is easily burnt. Antimony trioxide flame retardants provide a great opacifying effect due to their large diameter (0.5 to 10.0 .mu.m or so), their large refractive index and a small light transmittance. Therefore, the transparency which is the greatest feature of the halogen-containing vinyl resin will be completely lost if an antimony trioxide flame retardant is mixed with the resin.
In the paragraph bridging Cols. 1 and 2, it is explained that while antimony pentoxide is a flame retardant which does not cause the transparency of the resin to be lost, it worsens the stability of the resin. In the paragraph bridging Cols. 2 and 3, the cause of this worsening is explained. As stated therein, if antimony pentoxide is directly applied to provide flame retardancy, it will react with the metallic soap stabilizer which is usually used therein, e.g. zinc stearate, barium stearate or magnesium stearate which Watanabe states:
". . . which will outstandingly deteriorate its function as a stabilizer. Consequently, the plasticized halogen-containing vinyl resin will be worsened in thermal stability, and it can be presumed that the coloring and the bloom/bleed phenomena will be caused."
According to the invention described and claimed in Watanabe, these problems are solved by providing an antimony pentoxide powder having a particle size of 0.2 to 10 .mu.m (microns), which powder further contains specified amounts of (A) alkali metal oxide; (B) alkaline earth metal oxide; and (C) phosphoric acid, an ammonium salt of phosphoric acid, or an alkali metal salt of phosphoric acid.
It is stated in the patent that the flame retardant of the invention does not impair the transparency, the thermal stability, or the bleeding resistance of the halogen-containing vinyl resins which, as described in the patent, includig the illustrative examples, further contain the known heat stabilizers.
U.S. Pat. No. 3,897,389 issued to Touval Jul. 29, 1975 and assigned to M&T Chemicals, Inc., while apparently directed to reducing the antimony content while enhancing flame retarding efficiency alludes (Col. 1) to the fact that antimony trioxide "may be inappropriate in some applications where it is desired to have the final product in transparent form." In Col. 4 relating to test results, reference is made to a PVC specimen including, among other reagents, a heat stabilizer containing Ba/Cd/Zn and phosphorous, as well known, and stearic acid. According to the invention, flame retardancy may be provided to this resin as well as the others recited in the specification by incorporating therein a flame-retardant amount of sodium antimonate, the particles of which exhibit an average size of about 1.5 .mu.m, the size of the individual particles being between about 0.1 and 8 .mu.m.
Finally, reference is made to U.S. Pat. No. 4,741,865 issued to Kintz et al. May 3, 1988 and assigned to Nyacol Products, Inc., assignee of the present application.
In the paragraph bridging Cols. 2 and 3, it is stated that for many flame retardant applications, antimony pentoxide sols are dried to powders prior to use. It has been found that some polymer systems containing colloidal antimony pentoxide powder are degraded when cured by heating or otherwise exposed to elevated temperatures. Such degradation is evidenced by discoloration. It is then stated that this heat degradation can be alleviated by changing the surface of the antimony pentoxide particles to replace the hydronium ions on the surface with certain metal ions prior to spray drying. The required metals have essentially insoluble antimonates. Sodium, barium and calcium antimonates are insoluble, and solutions containing these metal ions are suitable for treating before drying. A convenient way to judge if an effective amount of metal substitution has been obtained is by the pH. A final pH of about 6.5 to 11 prior to drying indicates sufficient metal substitution. It is further stated that if the powder is to be used in a PVC plastisol, sufficient metal hydroxide (NaOH preferred) to provide a pH of 7.5 to 10 should be added to the antimony pentoxide sol before drying, with pH values of 8 to 9 being preferred. The patent claims all define the invention in terms of the pH, the convenient way to define the effective amount of metal substitution.!
In the examples, the antimony pentoxide flame retardant is added to a plastisol of PVC (dispersion of PVC in plasticizers) including barium, cadmium, zinc and phosphite heat stabilizers.
An embodiment of the invention described and claimed in the Kintz et al. patent described as a zero tint antimony pentoxide flame retardant powder is commercially available from PQ Corporation under the trade designation NYACOL ZTA. PQ's product sheet for NYACOL ZTA states that NYACOL ZTA has been specifically developed for flame retarding polyvinyl chloride. It has improved heat stability when used with Ba/Cd/Zn and phosphite type stabilizers.
In summary, all three of the above-mentioned patents and the commercial embodiment of the Kintz et al. patent have the common denominator in that their respective antimony pentoxide flame retardant compositions, when used with halogen containing vinyl resins having incorporated therein a heat stabilizer, provide the desired flame retardancy without degradation or adversely affecting the heat stability and optical qualities, e.g., the clarity or transparency of the resin.
Notwithstanding the fact that the aforementioned patented systems are said by the respective patentees to be capable of commercial acceptance and that the Kintz et al system assigned to the assignee of this application is in fact personally known to applicant to have achieved commercial success, there is always a continuing need for improvement of any product towards optimizing the product's acceptance, whether this be in performance or lowering manufacturing costs.
For this reason, Applicant engaged in a research effort at the R&D facility of Nyacol Products, Inc. with the view to improve the addition of a flame retardant to halogen-containing vinyl resins without any degradative effects.